When you search google looking for parts just type in Can Am OEM and it will bring up a number of sites where you can order parts from,
Can Am Parts Store :: Can AM OEM Parts :: Can Am ATV & SSV Online Store, this is one I have used it may be a little more expensive but they have everything. If you ordered stuff from BCS and it hasnt shipped yet call them I'm sure they have that gasket and can send it with your other stuff. As for coolant Engine Ice or Water Wetter will work fine, and If you never want to boil coolant at all go with Evan NPG-R it boils at 400 degrees at 5 psi, so you should technically never have to add more coolant.
For oil we use rotella t 15w-40 diesel oil, it works just as good as any oil and you can buy it by the gallon at walmart for 13 bucks, but really any motorcyle oil will do.